Change gfortran compile flags

gfortran now compiles with same flags CESM uses when DEBUG=TRUE, except I added
-Werror to elevate warnings to errors and then also turned on
-Wno-maybe-uninitialized to avoid failing tests because the compiler thinks
there's a chance we are accessing uninitialized memory (the one failure caught
by this in CI was initializing an allocatable array element-by-element inside a
loop: this was a false-positive, as the loop covered all indices)

bld/CompileFlags.mak

@@ -20,7 +20,7 @@ else
 endif
 
 ifeq ($(FC_TMP),gfortran)
-  FCFLAGS = -O2 -ffree-form -J $(OBJ_DIR) -cpp -Wall -Werror
+  FCFLAGS = -g -Og -Wall -Wno-maybe-uninitialized -Werror -fbacktrace -ffpe-trap=zero,overflow -fcheck=bounds -J $(OBJ_DIR)
 endif
 
 ifeq ($(FC_TMP),pgf90)